Introduction

A drug allergy is an allergic reaction to a medication. With an allergic reaction, your immune system, which fights infection and disease, reacts to the drug. This reaction can cause symptoms such as rash, fever, and trouble breathing.

True drug allergy is not common. Less than 5 to 10 percent of negative drug reactions are caused by genuine drug allergy. The rest are side effects of the drug. All the same, it’s important to know if you have a drug allergy and what to do about it.

Why do drug allergies happen?

Your immune system helps protect you from disease. It’s designed to fight foreign invaders such as viruses, bacteria, parasites, and other dangerous substances. With a drug allergy, your immune system mistakes a drug that enters your body for one of these invaders. In response to what it thinks is a threat, your immune system begins to make antibodies. These are special proteins that are programmed to attack the invader. In this case, they attack the drug.

This immune response leads to increased inflammation, which can cause symptoms such as rash, fever, or trouble breathing. The immune response might happen the first time you take the drug, or it may not be until after you’ve taken it many times with no problem.

Is a drug allergy always dangerous?

Not always. The symptoms of a drug allergy may be so mild that you hardly notice them. You might experience nothing more than a slight rash.

A severe drug allergy, however, can be life-threatening. It could cause anaphylaxis. Anaphylaxis is a sudden, life-threatening, whole-body reaction to a drug or other allergen. An anaphylactic reaction could occur minutes after you take the drug. In some cases, it could happen within 12 hours of taking the drug. Symptoms can include:

irregular heartbeat

trouble breathing

swelling

unconsciousness

Anaphylaxis can be fatal if it’s not treated right away. If you have any of the symptoms after taking a drug, have someone call 911 or go to the nearest emergency room.

Allergic-like reactions

Some drugs can cause an anaphylaxis-type reaction the first time they are used. Drugs that can cause a reaction similar to anaphylaxis include:

morphine

aspirin

some chemotherapy drugs

the dyes used in some X-rays

This type of reaction typically does not involve the immune system and is not a true allergy. However, the symptoms and treatment are the same as for true anaphylaxis, and it is just as dangerous.

What are the differences between side effects and a
drug allergy?

A drug allergy only affects certain people. It always involves the immune system and it always causes negative effects.

However, a side effect might occur in any person taking a drug. Also, it typically does not involve the immune system. A side effect is any action of the drug —harmful or helpful — that doesn’t relate to the drug’s main job.

For instance, aspirin, which is used to treat pain, often causes the harmful side effect of stomach upset. However, it also has the helpful side effect of reducing your risks of heart attack and stroke. Acetaminophen (Tylenol), which is also used for pain, can also cause liver damage. And nitroglycerin, which is used to widen blood vessels and improve blood flow, may improve mental function as a side effect.

How is a drug allergy treated?

How you manage a drug allergy depends on how severe it is. With a severe allergic reaction to a drug, you’ll likely need to avoid the drug entirely. Your doctor will probably try to replace the drug with a different one that you’re not allergic to.

If you have a mild allergic reaction to a drug, your doctor may still prescribe it for you. But they may also prescribe another medication to help control your reaction. Certain medications can help block the immune response and reduce symptoms. These include:

Antihistamines

Your body makes histamine when it thinks a substance, such as an allergen, is harmful. The release of histamine may trigger allergic symptoms such as swelling, itching, or irritation. An antihistamine blocks the production of histamine and may help calm these symptoms of an allergic reaction. Antihistamines come as pills, eyes drops, creams, and nasal sprays.

Corticosteroids

A drug allergy can cause swelling of your airways and other serious symptoms. Corticosteroids can help reduce the inflammation that leads to these problems. Corticosteroids come as pills, nasal sprays, eye drops, and creams. They also come as powder or liquid for use in an inhaler and liquid for injection or use in a nebulizer.

Bronchodilators

If your drug allergy causes wheezing or coughing, your doctor might recommend a bronchodilator. This drug will help open your airways and make breathing easier. Bronchodilators come in liquid and powder form for use in an inhaler or nebulizer.

What is the long-term outlook for someone with a drug allergy?

Your immune system can change over time. It’s possible that your allergy will weaken, go away, or get worse. So, it’s important always to follow your doctor’s instructions on how to manage a drug. If they tell you to avoid the drug or similar drugs, be sure to do so.